Security question: Example with myetherwallet.com private key

I'm taking a look about myetherwallet.com and i can see the following: When choose "Private key" on https://www.myetherwallet.com/#send-transaction , i just have to write a private key to see corresponding ETH and open a transaction form. Two questions about ethereum protocol (or myetherwallet.com mechanisms ?):

1. I can open any wallet and read how many ETH in it with a hex 64 characters string ? No need more "security" ?2. To make the transaction, what it will needed ? I think i am missing something ^^